The present invention relates to an improved method for accelerating the freezing of ice, initially formed by the freezing of a sea water spray or impounded sea water, and more particularly to an improved method to form an engineered load-bearing ice structure of high quality and in a shorter time than normally could be obtained.
Rapid freezing of sea water is important in certain applications such as the construction of load-bearing ice structures in offshore Arctic regions where such structures are employed in conjunction with hydrocarbon exploration and production and in the construction of airfields, roads, camps and the like. In these applications, sea water is used exclusively as the aqueous medium and construction is usually started as soon as the ambient air temperature is sufficiently low to cause freezing of the sea water. It is economically advantageous to be able to cause the freezing of sea water to proceed as rapidly as possible so that load-bearing structures may be constructed in a relatively short period of time so as to extend to the maximum degree possible the utility of the manufactured structure.
A method commonly employed to form ice structures involves the propelling of sea water through the air as essentially a stream of sea water and over significant horizontal distances. The volume of the continuous stream may range up to 30,000 gallons per minute from a single nozzle used to propel the salt water over the needed distance. The air, by virtue of its low temperature with respect to the nominal freezing temperature of sea water (-1.6 to -2.0 degrees C depending on salinity), acts as a coolant. The formation of droplets and the interaction of the sea water stream/droplet spray with cooler air results in freezing of the projected droplet spray. The efficiency of freezing depends on efficient heat exchange between the sprayed droplets and air. Formation of water droplets and the size of the droplets ultimately governs freezing efficiency at any ambient air temperature less than the nominal freezing temperature of the sea water. At the spray nozzle, the bulk of the sea water is in the form of a solid stream of water having high momentum in order to cover the desired relatively large horizontal distance. In the vicinity of the nozzle, shear and turbulent forces along the periphery of the water stream initiate droplet breakup and segregation. Along the trajectory of the stream/droplet spray, wind forces and gravitational forces promote increasing droplet breakup and segregation. Maximum droplet breakup, in the absence of significant wind forces, occurs at the apogee of the stream trajectory. The surface tension of the sea water is the fundamental property which governs how soon discrete water droplets will form and their size distribution for any imposed set of ambient conditions.
Load-bearing ice structures are also commonly built by forming a berm or dike and then flooding the impounded area with sea water, the process being repeated, after freezing of the sea water, as necessary until a desired thickness of ice has formed. Ice structures which are used as the support unit for large drill rigs are themselves large. Construction may require one or more months. It is necessary, therefore, to accelerate the ice construction phase so as to allow maximum time for drilling activities prior to the onset of the Spring thaw. The more or less routine application of flooding-spraying technology in conjunction with offshore Arctic application is described in the prior art, U.S. Pat. No. 4,048,808 being a typical example.
In accordance with this invention, it has been discovered that the governing property of a high volume sea water stream is formation of water droplets varying in a size from 1 to about 3 mm in diameter. These droplets freeze in the form of hailstones, which are rounded or spherical masses of ice. The interior of the frozen droplets commonly contain liquid water of high salinity consistent with finite freezing rates and thermodynamic constraints that govern the freezing of saline solutions which have a true eutectic. Successful ice construction requires that the projected sprayed material which falls to the surface have a liquid content. Some droplets crush on impact releasing additional brine. The fallen material undergoes partial melting and then refreezing. Excess brine drains either away from the structure by virtue of its reduced freezing temperature, caused by partial evaporation during flight and by salt rejection that occurs simultaneously with freezing or remains entrained in the porosity of the spray ice. On impact with the ground, the brine is released and there is some partial melting of the frozen material. The newly formed slush then refreezes upon exposure to ambient temperature air. The refreezing which occurs after impact is the phenomena that is responsible for strength development in sprayed ice.
In ice construction, where the aim is to build a substantial load-bearing structure of a relatively large dimension, dry snow is undesirable and detrimental because snow contributes to a general weakening of the manufactured structure and snow does not possess the substantial strength of ice.
Sea water spray construction of ice islands is a complex process that includes several important phenomena which collectively control the properties of the manufactured structure. Sea water is usually applied as a spray. The freezing of the spray is controlled by ambient climactic conditions, the volume of spray and the size distribution of water droplets within the spray. Spray ice, which consists of a mixture of ice and brine and/or precipitated salt may, depending upon ambient temperature and wind conditions, partially remelt upon impact and then slowly refreeze. Typically, spray ice construction is a cyclic process where sea water is sprayed for a period of time and then spraying is terminated to allow refreezing of the sprayed surface. The cycle is then repeated as necessary to produce the desired structure. Internal structure of spray ice reflects the cyclic nature of its formation.
Manufactured ice consists of alternating layers of relatively hard ice immediately underlain by a much thicker layer of much softer material. The internal structure of an ice island is a direct reflection of the techniques used for its construction.
The basic methodology for construction of an ice island using sea water spraying techniques, consists of freezing a sea water spray by the cooling action of ambient temperature air on the spray. Since sea water must be sprayed in large volumes over considerable horizontal distances, nozzles are selected primarily for their throwing or spraying distance. This requirement places rather stringent controls of the size of water droplets which form in the spray. It is the discrete water droplets which ultimately freeze and fall to the ground.
As droplets form in the spray, they freeze in the form of spherical hailstones consisting of ice. The cores of many of the larger hailstones contain brine significantly more saline than the source sea water due to partial evaporation of sprayed sea water and salt rejection during the freezing process. Upon impact, some hailstones shatter releasing brine. Depending upon ambient temperatures, some free, unfrozen brine may also reach the ground unfrozen but concentrated by partial evaporation. The spray may reach heights above ground surface of two hundred (200) feet or more. Air temperature differences between the maximum height attained by the spray and ground level can also encourage partial remelting of spray ice.
The saline brine contacts previously sprayed and frozen material and causes partial melting of this material. The residue brine as a consequence of the partial remelting decreases in salinity. The newly formed slush is then slowly refrozen by the action of the ambient air. The slush refreezes from its surface downward. As the initial upper surface refreezes, lower levels of the slush are insulated from direct air contact and they freeze at a lower rate. As a result of this process, the sprayed ice consists of cyclic deposits of hard ice immediately underlain by softer material that was prevented from fully freezing. If spraying is stopped and then resumed at a later time, the newly fallen material will cause partial remelting of the previously frozen surface. Thus, the thickness of the hard ice surface is probably never as great as it was when originally formed just before resumption of spraying.
A thermal gradient exists from the sea water-ice interface to the ice-air interface. Thermistor arrays are usually buried in an ice island during construction, and temperature data derived from these devices graphically demonstrate the heat transfer phenomena. Thus, partial remelting of newly formed spray ice is also a reflection of heat transfer from the warmer sea water to the colder free ice surface.
The primary factors that govern spray ice construction can be summarized as follows: (1) the freezing dynamics of a sea water spray, and (2) the refreezing of spray ice.
In the past, researches have concentrated on understanding spray freezing phenomena. Essentially, no attention has been devoted to the problem of spray ice refreezing. The dominating importance of spray ice refreezing can be readily understood when it is noted that during a typical twenty four (24) hour period, sea water may be sprayed for ten (10) hours or less whereas the remainder of the twenty four (24) hour period is spent waiting for spray ice to refreeze. Any improvement resulting in a diminution of the time required to refreeze spray ice may have dramatic and significant impact on overall construction time and cost.
The time required to refreeze spray ice after a spraying period is the major factor that influences the time required to build an ice structure. It would be desirable, therefore, to provide improved and relatively simple methods for accelerating spray ice refreezing.